This invention relates to a process for preparing ultrahigh molecular weight, high abrasion resistant polyethylene compositions.
Ultrahigh molecular weight linear polyethylene, having a viscosity average molecular weight of at least about 1,000,000 and a density of between about 0.92 and 0.99 has been known in the art for some time. These ultrahigh molecular weight polyethylene compositions have outstanding physical properties such as toughness, impact strength, abrasion resistance, low coefficient of friction as well as excellent resistance to attack by solvents and corrosive chemicals. Owing to these superior physical and chemical properties, these compositions are suitable for use in such demanding applications as vibration dampener pads, hydraulic cyclinders, mallet heads, flexible drive couplings, gears, belt and chain guides, etc.
It is known that ultrahigh molecular weight polyethylene can be cross-linked to still higher molecular weights by using free radical initiators. For example, U.S. Pat. Nos. 3,567,697 and 3,576,933 disclose a method for preparing melt shaped articles of cross-linked polyolefin compositions, such as polyethylene, wherein free radical initiators are used as crosslinking agents. These references teach that the polyethylene resin and free radical initiator must first be mixed, then melt shaped before cross-linking. The concentration of initiator is also substantially higher than the concentration needed to cross-link the resins according to the present invention.
Since ultrahigh molecular weight polyethylene compositions are extremely difficult to melt shape by conventional techniques such as injection molding, standard extrusion, thermoforming, etc., melt shaping is generally achieved by compression molding. Another technique sometimes used for meltshaping these high molecular weight compositions is ram extrusion wherein the resin is fed into one end of a straight tube, pushed through a heated zone and taken from the other end as a sintered rod or tube. Ram extrusion of ultrahigh molecular weight polyethylene in the presence of 0.2 to 0.6 weight percent organic peroxide is disclosed in U.S. Pat. No. 3,956,253, which is substantially in excess of the amount of initiator used herein. None of these references suggest that less than 0.100 weight percent of certain selected peroxides are suitable for preparing cross-linked ultrahigh molecular weight, high abrasion resistant polyethylene compositions.